I know that a list can be joined to make one long string as in:
x = ['a', 'b', 'c', 'd'] print ''.join(x)
Obviously this would output:
'abcd'
However, what I am trying to do is simply join the first and second strings in the list, then join the third and fourth and so on. In short, from the above example instead achieve an output of:
['ab', 'cd']
Is there any simple way to do this? I should probably also mention that the lengths of the strings in the list will be unpredictable, as will the number of strings within the list, though the number of strings will always be even. So the original list could just as well be:
['abcd', 'e', 'fg', 'hijklmn', 'opq', 'r']
Answers:
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Method 1
You can use slice notation with steps:
>>> x = "abcdefghijklm" >>> x[0::2] #0. 2. 4... 'acegikm' >>> x[1::2] #1. 3. 5 .. 'bdfhjl' >>> [i+j for i,j in zip(x[::2], x[1::2])] # zip makes (0,1),(2,3) ... ['ab', 'cd', 'ef', 'gh', 'ij', 'kl']
Same logic applies for lists too. String lenght doesn’t matter, because you’re simply adding two strings together.
Method 2
Use an iterator.
List comprehension:
>>> si = iter(['abcd', 'e', 'fg', 'hijklmn', 'opq', 'r']) >>> [c+next(si, '') for c in si] ['abcde', 'fghijklmn', 'opqr']
- Very efficient for memory usage.
- Exactly one traversal of s
Generator expression:
>>> si = iter(['abcd', 'e', 'fg', 'hijklmn', 'opq', 'r']) >>> pair_iter = (c+next(si, '') for c in si) >>> pair_iter # can be used in a for loop <generator object at 0x4ccaa8> >>> list(pair_iter) ['abcde', 'fghijklmn', 'opqr']
- use as an iterator
Using map, str.__add__, iter
>>> si = iter(['abcd', 'e', 'fg', 'hijklmn', 'opq', 'r']) >>> map(str.__add__, si, si) ['abcde', 'fghijklmn', 'opqr']
next(iterator[, default]) is available starting in Python 2.6
Method 3
just to be pythonic 🙂
>>> x = ['a1sd','23df','aaa','ccc','rrrr', 'ssss', 'e', ''] >>> [x[i] + x[i+1] for i in range(0,len(x),2)] ['a1sd23df', 'aaaccc', 'rrrrssss', 'e']
in case the you want to be alarmed if the list length is odd you can try:
[x[i] + x[i+1] if not len(x) %2 else 'odd index' for i in range(0,len(x),2)]
Best of Luck
Method 4
Without building temporary lists:
>>> import itertools >>> s = 'abcdefgh' >>> si = iter(s) >>> [''.join(each) for each in itertools.izip(si, si)] ['ab', 'cd', 'ef', 'gh']
or:
>>> import itertools
>>> s = 'abcdefgh'
>>> si = iter(s)
>>> map(''.join, itertools.izip(si, si))
['ab', 'cd', 'ef', 'gh']
Method 5
>>> lst = ['abcd', 'e', 'fg', 'hijklmn', 'opq', 'r'] >>> print [lst[2*i]+lst[2*i+1] for i in range(len(lst)/2)] ['abcde', 'fghijklmn', 'opqr']
Method 6
Well I would do it this way as I am no good with Regs..
CODE
t = '1. eat, foodn 7amn 2. brush, teethn 8amn 3. crack, eggsn 1pm'.splitlines() print [i+j for i,j in zip(t[::2],t[1::2])]
output:
['1. eat, food 7am', '2. brush, teeth 8am', '3. crack, eggs 1pm']
Hope this helps 🙂
Method 7
I came across this page interesting yesterday while wanting to solve a similar issue. I wanted to join items first in pairs using one string in between and then together using another string. Based on the code above I came up with the following function:
def pairs(params,pair_str, join_str): """Complex string join where items are first joined in pairs """ terms = iter(params) pairs = [pair_str.join(filter(len, [term, next(terms, '')])) for term in terms] return join_str.join(pairs)
This results in the following:
a = ['1','2','3','4','5','6','7','8','9'] print(pairs(a, ' plus ', ' and ')) >>1 plus 2 and 3 plus 4 and 5 plus 6 and 7 plus 8 and 9
The filter step prevents the ” which is produced in case of an odd number of terms from putting a final pair_str at the end.
All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0